Aqueous emulsion of fluorocarbon polymer and method for producing the same

ABSTRACT

An aqueous emulsion of fluorocarbon polymer according to the present invention is characterized by that the emulsion comprises a sodium dialkylsulfosuccinic acid of not less than 1.5% by weight relative to the fluorocarbon polymer, and an alkyleneglycol. A method for producing a stabilized aqueous emulsions of fluorocarbon polymer according to the present invention comprises mixing an aqueous emulsion of fluorocarbon polymer obtained by an emulsion polymerization process with a sodium dialkylsulfosuccinic acid dissolved in an alkyleneglycol or an aqueous solution of the alkyleneglycol. A novel aqueous emulsion of fluorocarbon polymer having excellent mechanical stability and thermal stability is obtainable.

This application claims benefit of international application PCT/JP95/01325 filed Jul. 3, 1995.

TECHNICAL FIELD

The present invention relates to a novel aqueous emulsion offluorocarbon polymer having excellent mechanical stability and thermalstability, and to a method for producing the same.

BACKGROUND ART

Aqueous emulsion of polytetrafluoroethylene (PTFE) are prepared by theemulsion-polymerization method disclosed in U.S. Pat. No. 2,559,752. Inthe method, tetrafluoroethylene is charged under pressure into anaqueous medium containing a water-soluble polymerization initiator andan anionic surfactant having a fluoroalkyl hydrophobic group to resultin formation of colloidal particles of PTFE in the medium. The aqueousemulsion of other fluorocarbon polymers also are prepared by similarmethods. The aqueous emulsion of fluorocarbon polymer thus prepared arestabilized by addition of emulsion stabilizers because of inferiormechanical stability of the emulsion itself.

For the emulsion stabilizer, hydrocarbon-base surfactants are employedusually in place of fluorocarbon-base surfactants mainly from theeconomical viewpoint, and commonly used emulsion stabilizers includesuch nonionic surfactants as p-C₈₋₁₀alkylphenylpolyethyleneglycolethers. However, emulsions thus stabilizedby such surfactants are still not satisfactory in stability, and tend toturn into unstable ones by their encounter with mechanical actionsduring evaporation, concentration, dilution, transportation, weighingand others.

Anionic surfactants are also proposed for use as the emulsionstabilizer, and, for example, U.S. Pat. No. 4,369,266 mentionsextensively such anionic surfactants as esters or salts of alkylsulfonicacids, alkylsulfuric acids, alkylarylsulfonic acids, alkylaryl sulfuricacids, higher fatty acids, alkyl phosphoric acid esters, alkylarylphosphoric acid esters and sulfosuccinic acid. However, commercialusages of these surfactants for emulsion stabilizers are scarcelyrecognized, and further no mention is made in Examples of the U.S.Patent as to those anionic surfactants being used as emulsionstabilizers.

It is an object of the present invention to provide an aqueous emulsionof fluorocarbon polymer having excellent mechanical stability. By use ofthe present emulsion, for example, prevention becomes feasible againstsuch problems caused by inferior mechanical stability as clogging ofpumps, valves and nozzles with coagulated colloidal particles offluorocarbon polymer formed during agitation, transportation andspraying, adhesion of coagulated particles to container walls andagitators, and flotation of coagulated particles in the emulsion. It isanother object of the present invention to provide an aqueous emulsionof fluorocarbon polymer having excellent thermal stability. By use ofthe present emulsion, for example, prevention becomes feasible againstsuch problems caused by inferior thermal stability as deterioratedcoating and impregnation properties towards metals, glass woven fabricbecause of increased emulsion viscosities under high temperatures. It isa further object of the present invention to provide a method forproducing efficiently in a short period of time an aqueous emulsion offluorocarbon polymer having excellent mechanical and thermalstabilities.

For the purpose of obtaining aqueous emulsions of fluorocarbon polymerhaving excellent mechanical and thermal stabilities, the presentinventors conducted numerous experiments using prospective surfactantsincluding the aforementioned as the emulsion stabilizer. The resultshowed that most of the surfactants were not suitable due to causing,for example, inferior emulsion stability, excessive foaming of emulsionand coloration of coated glass woven fabric owing to surfactants,however, sodium salt of a specified alkylsulfosuccinic acid was found tobe an effective emulsion stabilizer for a low content aqueous emulsionof fluorocarbon polymer. As the result of further studies, successfulstabilization of aqueous emulsions of fluorocarbon polymer under theirhigh content regions was obtained by use of sodium dialkylsulfosuccinicacids as the emulsion stabilizer to accomplish the present invention.Further, a method for producing efficiently aqueous emulsions offluorocarbon polymer according to the present invention was achievedsuccessfully.

DISCLOSURE OF THE INVENTION

An aqueous emulsion of fluorocarbon polymer according to the presentinvention is characterized by that the emulsion comprises a sodiumdialkylsulfosuccinic acid of not less than 1.5% by weight relative tothe fluorocarbon polymer, and an alkyleneglycol. A method for producinga stabilized aqueous emulsions of fluorocarbon polymer according to thepresent invention comprises mixing an aqueous emulsion of fluorocarbonpolymer, obtained by an emulsion polymerization method with a sodiumdialkylsulfosuccinic acid dissolved in an alkyleneglycol or an aqueoussolution of the alkyleneglycol.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the fluorocarbon polymer includes polymers oftetrafluoroethylene, chlorotrifluoroethylene or vinylidenefluoride, orcopolymers containing the aforementioned. Mention is made of thepolymers including PTFE, tetrafluoroethylene.hexafluoropropylenecopolymer, tetrafluoroethylene.fluoroalkylvinylether copolymer,tetrafluoroethylene.ethylene copolymer, polychlorotrifluoroethylene,polyvinylidenefluoride and vinylidenefluoride.hexafluoropropylenecopolymer.

In the present invention, the aqueous emulsion means those containing inwater 1-75 wt % of the fluorocarbon polymer in colloidal particleshaving an average particle size of 0.1-0.3μ. The average particle sizeof colloidal particles is measured by the centrifugal sedimentationmethod. In the present invention, the values were measured by use of acentrifugal sedimentation particle size distribution measuring equipment(SA-CP4L; Shimadzu Corp.).

For the emulsion stabilizer, sodium dialkylsulfosuccinic acids areemployed. The alkyl radical has 8-12 carbon atoms, and octyl or nonylradical is specifically preferred. Such emulsion stabilizers werealready employed for aqueous emulsions of PTFE, however, theconcentrations of PTFE as well as of the emulsion stabilizer in theaqueous emulsion were quite low. For example, though U.S. Pat. No.2,478,229 employs in Example 4 sodium dioctylsulfosuccinic acid which isthe emulsion stabilizer of aqueous emulsions of the present invention,since the concentration of PTFE emulsion is so low as 3.2 wt % and 1 wt% solution of the above emulsion stabilizer is added thereto, theconcentration of resulting stabilized emulsion is lowered further to2.78 wt %. That is, it was a problem to require quite a long period oftime to obtain a concentrated aqueous PTFE emulsion by adding directlythe emulsion stabilizer to the aqueous emulsion for stabilizationthereof, as the solubility to water of this kind of emulsion stabilizersis so low as around 1.5% that it takes a too long time for dissolutionof the stabilizer into emulsion to achieve stabilization of theemulsion. Accordingly, it was difficult to prepare industriallyconcentrated aqueous PTFE emulsions having a concentration larger than10 wt %.

For preparing in a short period of time a stabilized concentratedaqueous PTFE emulsion, the present inventors have found it effective tomix an aqueous PTFE emulsion with a sodium dialkylsulfosuccinic aciddissolved beforehand in an alkyleneglycol especially in ethyleneglycolor propyleneglycol or in an aqueous alkyleneglycol solution. In theabove, the alkyleneglycol serves for sodium dialkylsulfosuccinic acidsas a solubilizer towards water. On the other hand, when carbontetrachloride, petroleum ether, naphtha, xylene, petroleum-basesolvents, acetone, alcohols, etc. known as organic solvents for sodiumdialkylsulfosuccinic acids were used as the solubilizer, the resultingaqueous PTFE emulsions were unstable.

The amount of sodium dialkylsulfosuccinic acid added is not less than1.5%, preferably 2-5%, by weight of the fluorocarbon polymer. When theamount is less than 1.5%, the emulsion tends to become unstable. Nospecific upper limit exists for the amount, however, an amount over 5%is uneconomical. The amount of alkyleneglycol is so settled as toimprove the solubility of sodium dialkylsulfosuccinic acid in water butbe not too much to render the aqueous emulsion of fluorocarbon polymerunstable. The more the amount of alkyleneglycol, the more the solubilityof sodium dialkylsulfosuccinic acid in water is improved, however, toomuch amount makes the aqueous emulsion of fluorocarbon polymer unstable.Thus, the amount of alkyleneglycol added is not more than 10%,preferably not more than 5%, by weight of the fluorocarbon polymer. Nospecific lower limit exists for the amount of alkyleneglycol, however,an amount over 1%, preferably over 5%, more preferably over 10%, byweight of the sodium dialkylsulfosuccinic acid is recommended.

In the method for producing the aqueous emulsion of fluorocarbonpolymer, it is important to provide beforehand a solution of sodiumdialkylsulfosuccinic acid in an alkyleneglycol or its aqueous solution,and to mix the solution with an aqueous fluorocarbon polymer emulsion toobtain an emulsion according to the present invention. If the sodiumdialkylsulfosuccinic acid is firstly mixed with an aqueous fluorocarbonpolymer emulsion and then an alkyleneglycol is added, or is mixed with amixture of an aqueous fluorocarbon polymer emulsion and analkyleneglycol, the sodium dialkylsulfosuccinic acid encounters withdifficulty in dissolving into the mixture.

The mechanical stability of the aqueous emulsion of fluorocarbon polymercan be assessed by observation of increased ratio of apparent sizes ofassociated colloidal particles resulting from collisions of particles inthe emulsion during high speed agitation of the emulsion. Further, underagitation, the colloidal particles coagulate partly to form a coagulum,which may adhere to agitator rotors, vessel walls and others, or isolatefrom the emulsion as a flock. Under occurrence of these phenomenon, theemulsion is regarded as having an inferior stability, despite of theincrease or decrease in size of colloidal particles remaining in theemulsion. In the present invention, the mechanical stability is assessedby increased ratio of particle size measured according to the followingmethod. Into a beaker of 60 mm inner diameter is charged 200 ml of anemulsion having an average particle size (S₁). An agitator(ULTRA-TURRAX; JUNKE & KUNKEL GMBH & Co. KG) having a shaft with agenerator is so put in the beaker as the shaft is apart by 15 mm fromthe bottom of beaker and deviates by 5 mm from the center, and theagitator is operated for 5 minutes under 20,500 rpm. The averageparticle size (S₂) of emulsion after being subjected to the agitation ismeasured, and the increased ratio of particle size is calculatedaccording to the following formula:

Increased ratio of particle size (%)=(S₂ -S₁)/S₁ 33 100

The mechanical stability is rated under the following standard:

Good: Increased ratio of particle size is >0%-<5%;

Fair: Increased ratio of particle size is ≧5% -<10%; and

Poor: Increased ratio of particle size is ≧10%.

In addition to the above, Poor is rated when a large amount ofcoagulated matter is formed by the agitation.

Viscosity of the aqueous emulsion increases in accordance with increasein the temperature of emulsion. Unfavorable emulsions show sharpincrease in viscosity at 30°-50° C. the emulsions encounter when storedin summer. In the present invention, the viscosity was measured at20°-60° C. for aqueous emulsions containing about 60 wt % fluorocarbonpolymer by use of a B-Type viscometer (Type BL; Tokyo Keiki). Emulsionsshowing vigorous increases in viscosity at 30°-50° C. are recognized ashaving poor thermal stability.

Examples of the aqueous emulsion of fluorocarbon polymer will beexplained hereunder, in which aqueous emulsions of PTFE were employed asthe emulsion since they were regarded as the most difficultlystabilized.

Example 1!

Mechanical Stability Test: To 300 ml of an aqueous emulsion containingabout 45wt % PTFE , obtained by the emulsion polymerization method wasadded sodium dioctylsulfosuccinic acid dissolved in 50wt% aqueoussolution of propyleneglycol by amounts mentioned in Table 1, and theresulted mixture was stirred (100 rpm) in a 500 ml beaker for 5 minunder addition of water to prepare 30 wt % PTFE aqueous emulsion sampleA. The sample was subjected to the measurement of increased ratio ofparticle size according to the method mentioned previously. The resultshown in Table 1 indicates that the increased ratio of particle size wasquite small.

                  TABLE 1                                                         ______________________________________                                        sodium                                                                        dioctyl     aqueous solution                                                                           average    increased                                 sulfo-      of propyleneglycol                                                                         particle size                                                                            ratio of                                         succinic propylene                                                                              water before                                                                              after                                                                              average                                    acid     glycol   (wt % agita-                                                                              agita-                                                                             particle                                   (wt % per                                                                              (wt % per)                                                                             per   tion  tion size                                Sample PTFE)    PTFE)    PTFE) (μ)                                                                              (μ)                                                                             (%)                                 ______________________________________                                        A      3.0      0.5      0.5   0.238 0.241                                                                              1.3                                 (Example)                                                                     ______________________________________                                    

Example 2!

Mechanical Stability Test: To 300 ml of an aqueous emulsion containingabout 45 wt % PTFE , obtained by the emulsion polymerization method wasadded sodium dioctylsulfosuccinic acid dissolved in an alkyleneglycol byamounts mentioned in Table 2, and the resulted mixture was stirred (100rpm) in a 500 ml beaker for 5 min under addition of water to prepare30wt % PTFE aqueous emulsion samples B, C, D and E. Each sample wassubjected to the measurement of increased ratio of particle sizeaccording to the method mentioned previously. The result is shown inTable 2.

Comparative Example 1!

Samples F and G were prepared in the same manner as Example 1 except forusing kerosene or alcohols as the solvent for sodiumdioctylsulfosuccinic acid, and the increased ratio of particle size wasmeasured. The result is shown in Table 2.

As understandable from Table 2, samples B-E containing sodiumdioctylsulfosuccinic acid and alkyleneglycol show smaller increasedratio of particle size for the PTFE aqueous emulsions than those ofsamples F and G using kerosene or alcohols as solvent for the salt,which indicates an improved mechanical stability is obtainable.

                                      TABLE 2                                     __________________________________________________________________________    sodium                                                                        dioctyl                average increased                                      sulfo-                 particle size                                                                         ratio of                                             succinic                                                                           propylene                                                                            ethylene                                                                           before                                                                             after                                                                            average                                              acid glycol glycol                                                                             agita-                                                                             agita-                                                                           particle                                             (wt % per                                                                          (wt % per                                                                            (wt % per                                                                          tion tion                                                                             size                                           Sample                                                                              PTFE)                                                                              PTFE)  PTFE)                                                                              (μ)                                                                             (μ)                                                                           (%)                                            __________________________________________________________________________    B     3.0  1.0    --   0.220                                                                              0.223                                                                            1.4                                            (Example)                                                                     C     3.0  3.0    --   0.224                                                                              0.228                                                                            1.8                                            (Example)                                                                     D     3.0  0.5    1.0  0.229                                                                              0.231                                                                            0.9                                            (Example)                                                                     E     3.0  --     3.0  0.227                                                                              0.233                                                                            2.6                                            (Example)                                                                     F     3.0  kerosene                                                                             2.0  0.233                                                                              0.251                                                                            7.7                                            (Comp.                                                                        Example)                                                                      G     3.0  isopropanol                                                                          1.5  0.233                                                                              unmeasurable                                      (Comp.     methanol                                                                             1.5                                                         Example)                    (coagulation)                                     __________________________________________________________________________

Comparative Example 2!

Except that reagent grade sodium dioctylsulfosuccinic acid powder (noaddition of alkyleneglycol) was added by 3.0% by weight of PTFE to anaqueous emulsion containing about 45 wt % PTFE, obtained by the emulsionpolymerization method and that the period of time for stirring (100 rpm)was altered, 30 wt % aqueous PTFE emulsions were prepared in the samemanner as the Examples. Measurements of increased ratio of particle sizewere conducted for the samples having stirring period of 0.5, 1, 3 and 8hr, and the result is shown in Table 3. In the mechanical stabilitytest, the sample having 0.5 hr stirring showed coagulation, sampleshaving 1 hr and 3 hr stirring were inferior to Examples of the inventionand only the sample having 8 hr stirring showed equal result.

                  TABLE 3                                                         ______________________________________                                                      average   increased                                                           particle size                                                                           ratio of                                                                before     after                                                                              average                                               stirring                                                                              agita-     agita-                                                                             particle                                              period  tion       tion size                                        Sample    (hr)    (μ)     (μ)                                                                             (%)                                         ______________________________________                                        H         0.5     0.234      unmeasurable                                     (Comp.                       (coagulation)                                    Example)                                                                      I         1       0.234      0.247                                                                              5.6                                         (Comp.                                                                        Example)                                                                      J         3       0.234      0.241                                                                              3.0                                         (Comp.                                                                        Example)                                                                      K         8       0.234      0.236                                                                              0.9                                         (Comp.                                                                        Example)                                                                      ______________________________________                                    

Example 3 and Comparative Example 3!

Sample L prepared by concentrating Sample A to 59.1 wt % content, andsample M being 60 wt % PTFE aqueous emulsion containingp-nonylphenylpolyethyleneglycolether (poly=9) by 3.0% by weight of PTFEwere subjected to the thermal stability test, and the result is shown inTable 4. Sample M shows a rapid increase in viscosity at above 40° C.,but sample L being an example of the invention does a slow increase.Increased particle size for each sample was 3.4% and 5.4% respectively.

                  TABLE 4                                                         ______________________________________                                        kind of                                                                       emulsion     Viscosity (cP)                                                   Sample stabilizer                                                                              20° C.                                                                         30° C.                                                                       40° C.                                                                       50° C.                                                                       60° C.                      ______________________________________                                        L      sodium    22.5    25.2  25.8  31.7  34.6                               (Example)                                                                            dioctylsulfo-                                                                 succinic                                                                      acid                                                                   M      p-nonyl-  22.5    21.5  22.5  56.0  97.0                               (Comp. phenyl poly-                                                           Example)                                                                             ethylene-                                                                     glycol                                                                        ether                                                                         (poly = 9)                                                             ______________________________________                                    

Example 4 and Comparative Example 4!

Samples of 30% PTFE aqueous emulsion N, O, P, Q and R containingdifferent amount of sodium dioctylsulfosuccinic acid as mentioned inTable 5 were prepared in a similar manner as that of Example 1, and theincreased ratio of particle size was measured. The result is shown inTable 5. The sample containing sodium dioctylsulfosuccinic acid by0.5%(Q) or 1%(R) shows coagulation of the emulsion.

                  TABLE 5                                                         ______________________________________                                        sodium                                                                        dioctyl                average     increased                                  sulfo-                 particle size                                                                             ratio of                                          succinic   propylene                                                                              before after                                                                              average                                       acid       glycol   agita- agita-                                                                             particle                                      (wt % per  (wt % per                                                                              tion   tion size                                   Sample PTFE)      PTFE)    (μ) (μ)                                                                             (%)                                    ______________________________________                                        N      3.0        3.0      0.224  0.228                                                                              1.8                                    (Example)                                                                     O      2.2        0.4      0.224  0.225                                                                              0.9                                    (Example)                                                                     P      1.5        0.3      0.232  0.238                                                                              2.6                                    (Example)                                                                     Q      1.0        0.2      0.232  unmeasurable                                (Comp.                            (coagulation)                               Example)                                                                      R      0.5        0.1      0.232  unmeasurable                                (Comp.                            (coagulation)                               Example)                                                                      ______________________________________                                    

Example 5!

Samples S, T, U and V were prepared by diluting 30% PTFE aqueousemulsions N and O shown in Table 5 with water by 10 or 30 times, and theincreased ratio of particle size for the diluted samples were measured.The result is mentioned in Table 6. Aqueous emulsions N and O show smallincreased ratios of particle size and good mechanical stability withoutconcern in the degree of dilution with water.

                  TABLE 6                                                         ______________________________________                                        sodium                                                                        dioctyl                      average  increased                               sulfo-                       particle size                                                                          ratio of                                       succinic propylene      before                                                                              after                                                                              average                                    acid     glycol         aita- agita-                                                                             particle                                   (wt % per                                                                              (wt % per                                                                              dilution                                                                            tion  tion size                                Sample PTFE)    PTFE)    ratio (μ)                                                                              (μ)                                                                             (%)                                 ______________________________________                                        S      3.0      3.0      10    0.226 0.230                                                                              1.8                                 (Example)                                                                     T      3.0      3.0      30    0.224 0.225                                                                              0.9                                 (Example)                                                                     U      2.2      0.4      10    0.224 0.224                                                                              0.0                                 (Example)                                                                     V      2.2      0.4      30    0.223 0.225                                                                              0.9                                 (Example)                                                                     ______________________________________                                    

INDUSTRIAL APPLICABILITY

Thanks to the present invention, it has become possible to produceefficiently in a short period of time PTFE aqueous emulsions havingsuperior mechanical stability and thermal stability. By use of suchsuperior properties of fluorocarbon resins exhibited in the surfaceproperty, heat resistance, chemical resistance and electrical property,the aqueous emulsions of fluorocarbon polymer according to the presentinvention are employed as coating agents for metals or the like,impregnating agents for fibers and cloths, additives for variousmaterials including such thermoplastics as polycarbonate to make themflame retardant, and further as dust-preventive treating agents. In theapplication field of aqueous emulsion of fluorocarbon polymer, therewere tendencies due to insufficient mechanical and thermal stability ofthe emulsion that the emulsion became unstable during evaporation,concentration, dilution, transportation, weighing in the productionprocess and during coating, impregnation, mixing in the applicationprocess, which were serious problems to be solved. Though it wasdifficult to obtain efficiently in a short period of time aqueousemulsions of fluorocarbon polymer having commercially sufficientstability, the present invention achieved to provide an aqueous emulsionof fluorocarbon polymer capable of solving these problems, and further amethod for producing the emulsion.

What is claimed is:
 1. An aqueous emulsion of fluorocarbon polymercomprising:a fluorocarbon polymer selected from the group consisting ofpolymers of tetrafluoroethylene, chlorotrifluoroethylene andvinylidenefluoride, or a fluorocarbon copolymer selected from the groupconsisting of tetrafluoroethylene.hexafluoropropylene copolymer,tetrafluoroethylene.fluoroalkylvinylether copolymer,tetrafluoroethylene.ethylene copolymer,polychlorotrifluoroethylene.hexafluoropropylene copolymer,polyvinylidenefluoride.hexafluoropropylene copolymer, endvinylidenefluoride.hexafluoropropylene copolymer; sodiumdialkylsulfosuccinic acid of not less than 1.5% by weight relative tothe fluorocarbon polymer; and an alkyleneglycol.
 2. An aqueous emulsionof fluorocarbon polymer according to claim 1, in which the content ofalkyleneglycol is not less than 1% by weight relative to the sodiumdialkylsulfosuccinic acid.
 3. A method for producing stabilized aqueousemulsions of fluorocarbon polymer, wherein said method comprising:a stepof mixing aqueous emulsions of a fluorocarbon polymer obtained by anemulsion polymerization process with a sodium dialkylsulfosuccinic aciddissolved in an alkyleneglycol or an aqueous solution of thealkyleneglycol, said fluorocarbon polymer or copolymers thereof beingselected from the group consisting of polymers of tetrafluoroethylene,chlorotrifluoroethylene and vinylidenefluoride, and a fluorocarboncopolymer selected from the group consisting oftetrafluoroethylene.hexafluoropropylene copolymer,tetrafluoroethylene.fluoroalkylvinylether copolymer,tetrafluoroethylene.ethylene copolymer,polychlorotrifluoroethylene.hexafluoropropylene copolymer,polyvinylidenefluoride.hexafluoropropylene copolymer, andvinylidenefluoride.hexafluoropropylene copolymer.